ABSTRACT The aims of this study are to define the structural features of allosteric sites on muscarinic acetylcholine receptors, as well as the regions of the receptor that regulate the cooperativity between allosteric and orthosteric sites. Muscarinic receptors have become a key model system for the molecular study of allosteric drug action at G protein-coupled receptors. Therefore, the knowledge to be gained from the proposed experiments is expected to have benefits ranging far beyond the advancement of muscarinic pharmacology. Allosteric drugs have inherent advantages of selectivity, efficacy, and safety. These advantages are especially important when the desired effect is to enhance the action of a neurotransmitter at a particular receptor in the brain. Allosteric enhancers are able to preserve the pattern of activity that is dictated by the very complex spatiotemporal motif of synaptic activity; by contrast, directly-acting agonists tend to disrupt these complex motifs that are the hallmark of synaptic activity in the brain. Furthermore, it is now becoming clear that there are multiple allosteric sites on muscarinic receptors. The usual case is that an allosteric ligand binds to its specific site and regulates the affinity with which acetylcholine binds to its (orthosteric) site on the receptor. This results in either a gain or loss in the potency of acetylcholine. We have recently identified a novel allosteric site, through which specific ligands have the unusual effect of being able to regulate the ability of acetylcholine to activate the receptor once it is bound, while altering potency only slightly or not at all. We have found that the sites for these different types of ligands are distinct and can be activated simultaneously. Furthermore, specific combinations of ligands depress low levels of response to neurotransmitter and enhance maximal response, or vice versa; we call this effect signal shaping. Additionally, we have designed tools and techniques that are sensitive to the molecular nature of the receptor and are calibrating them against verified monomeric and oligomeric receptors. We will use these tools to test our model that the functional receptor unit in tissues is an oligomer. Finally, we will label the best known muscarinic allosteric site with an irreversible ligand and determine which residue is covalently labeled through the use of affinity chromatography, HPLC, and mass spectrometric methods. We expect these studies to lead to improvements in the cholinergic pharmacology of disorders of the central nervous system, including Alzheimer's disease, and to contribute to an understanding of the potential for allosteric modulation of other G protein-coupled receptors.